Three detector cycle counting means



Sept. 6, 1966 A. K. CHITAYAT 3,271,676

THREE DETECTQR CYCLE COUNTING MEANS Original Filed Jan. 25, 1965 5Sheets-6 1 T l FORWARD REVERSE B INVENTOR.

ANWAR K.CHITAYAT BY Sept. 6, 1966 A. K- CHITAYAT THREE DETECTOR CYCLECOUNTING MEANS Original Filed Jan. 23, 1963 FORWARD q me A: -ETB; B: P-P

J -Bg+A v Mon $UBTRACT 5 Sheets-Sheet 5 REVERSE G INVENTOR.

ANWAR K. CHITAYAT United States Patent 3,271,676 THREE DETECTOR CYCLECOUNTING MEANS Anwar K. Chitayat, Plainview, N.Y., assignor toQptomechanisms, Inc, Plainview, NRY. Original application Jan. 23, 1963,Ser. No. 253,473. Divided and this application Jan. 25, 1963, Ser. No.

3 Claims. (Cl. 32483) This invention relates to three detector cyclecounting means and more particularly to means for counting fractionalcycles of phase shift.

This application is a division of my co-pending apphcation entitledAutomatic Measuring Interferometer, Serial No. 253,473 filed: January23, 1963.

The invention is illustrated in connection with an improvement of theMichelson type interferometer. Means are provided to produce threevariable waves which differ 120 in phase. The fringes produced by thesewaves and a reference wave are separated and picked-up by threedetectors. The outputs of the detectors are combined so as to providedirection sensitive counting means for counting the fringes and therebymeasure the position of a movable member with an accuracy of of thefringe wavelength.

Accordingly, a principal object of the invention is to provide new andimproved cycle counting means.

Another object of the invention is to provide new and improved automaticcounting means.

Another object of the invention is to provide new and improved means forprecise measurement.

Another object of the invention is to provide new and improved means forprecise measurement using light waves and automatic direction sensitivecounting means.

Another object of the invention is to provide new and improved means formaking precision measurements having three phase detector means.

Another object of the invention is to provide new and improved means formaking automatic precision measurements of the order of one-twelfth of awave.

Another object of the invention is to provide new and improved fringecounting means.

Another object of the invention is to provide new and improved addingsubtracting fringe counting means.

These and other objects of the invention will be apparent from thefollowing specification and drawings of which:

FIGURE 1 is a schematic plan view of part of the embodiment of theinvention.

FIGURE 2 is a block diagram of the circuit of the embodiment of theinvention.

FIGURES 3, 3a, 4, and 4a, are diagrams illustrative of the operation ofthe invention.

Referring to FIGURE 1, there is shown a modification of a Michelson typeinterferometer. Here, a monochromatic light source S is used with anaperture T. The illumination is collimated by lens L and is directedonto the beam splitter G. The illumination is then separated in twoparts:

(I) to a fixed position tetrahedral prism C which is then reflected backto beam splitter G, at which a portion is reflected to the lens L and(II) to a tetrahedral prism (C mounted on movable member X and back tobeam splitter G, and then to the lens L The compensating plate E isadded by the present invention as will be described.

All of the above elements are preferably mounted on a glass plate B,which is preferably at least one-half inch thick. The elements may bemounted in a conventional manner for instance, by cementing onto theglass plate.

If desired, adjustable mountings may be provided in conventional manner.

The light source S may be a Hg-l98 source having a wavelength of 5461angstroms which provides fringes approximately ten millionths of aninch.

In the Michelson type interferometer fringes are formed by theinteraction of the light (I) and (II) such that the amplitude seen atlens (L can be the summation of the illumination through (I) and (II) asa maximum, and their difference as a minimum.

The type of the interferometer described above has been used in manyapplications where automatic measurement is desired. However, means mustbe added to detect direction of movement and in addition to develop acounting system which is insensitive to light level changes and contrastas well as amplifier sensitivity.

The following is a description of a unique detection scheme whichresults in the following:

(I) Direction sensing.

(II) A sensor whose operation is independent of slight variation ofillumination level, or contrast.

(III) A scheme whereby fractions of a fringe can readily be counted toas low as of a fringe, with a resultant increase in sensitivity.

FIGURE 1 illustrates the arrangement of the sensor means. Multilayercoatings 1-6 are deposited on the compensating glass plate E. Thecoatings may be magnesium fluoride or silicon monoxide. The coatings areapplied so as to retard the light so that it changes optical path in thefollowing manner.

Areas 1 and 6: L/ 6 retardation Areas 2 and 5 L/ 3 retardation Areas 3and 4: No retardation Where L=wavelength The illumination enters thecompensating plate twice. Thus, if it enters at Area 1, it is directedto the tetrahedral prism C and is reflected back at Area 6. In thesamemanner, the beam entering Area 2, passes through Area 5 in its returnpath. Also, that entering Area 3 exits through Area 4.

Consequently, any beam entering through any of the areas results in adouble path retardation in the following manner.

Areas 1 and 6: L/ 3 retardation Areas 2 and 5 2L/ 3 retardation Areas 3and 4: No retardation.

It may be noted from the above that if one replaces the tetrahedralprisms by mirrors, the same retardations will result as shown above,except that the illumination reflects back in the same area twice.Consequently, the system described above is equally suited for mirrorsas for prisms. However, if mirrors only are used, only three areas aresuflicient instead of six.

Based on the above, it may be seen that if one moves the tetrahedralprism C in a reverse direction (toward B) at a constant velocity, thenthree sine wave shapes are developed as shown in FIGURE 3A; where Prepresents areas 1 and 6; P represents areas 2 and 5; and P represent-sareas 3 and 4. The three areas cause a phase shift, so that P lags P by/s of one fringe; and P lags P by 240=% of one fringe.

The three areas are detected by three separate photomultipliers as shownin FIGURE 1. M reflects the illumination or areas 3 and 4 onto thephotomultiplier F having output P M reflects areas 2 and 5 onto thephotomultiplier F having output P Photomultiplier F having output Pdetects the illumination of areas 1 and 6.

In order to allow for the sensing of direction, it is desirable toobtain two electronic signals A and B which are 90 out of phase. This isachieved in the following manner. The output of the photomultiplier F issubtracted from P through a difference amplifier to result in B: (P2P3).While the quadrature signal A is obtained by the summation It is shownin FIGURE 3A(b) that these signals are out of phase by 90.

In the above discussion, it was assumed that only A.C. signals aredeveloped. However, in actual practice, it is not possible to obtainperfect contrast, so that the lowest signal is not zero but is abovezero. In other words, it is possible to obtain a DC. level which isadded to all the other signals. This is caused by imperfection ofoptics, poor monochromatieity of the light source, or the introductionof background illumination. In any case, it is important to minimize theeffects of this D.C. level which may vary in time and on the order ofinterference. The scheme described above allows the cancellation of theDC. level. Thus, (P P signal is independent of the DC. level that isadded to both P and P equally. In addition, the

signal is also independent of the DC. levels added to P P and P equally.

FIGURES 2, 3, and 3A illustrate the principle of operation of thedetection system. The outputs from pickups F F and F are fed toamplifiers R R and R Summation network provides the outputs and B=P -P Atypical summation network is shown in Radiation Laboratory Series volume19, chapter 18, section 2, published by McGraw-Hill. These signalsrepresent the position of the moving tetrahedral prism C Now, if C ismoved at a constant velocity, then alternating signals are developed,the sinusoidal response of the photomultipliers being shown in FIGURE 3.If the direction of movement is forward, P lags P which in turn lags Pby L/3=120 intervals.

On the other hand, if the direction is reversed, FIG- URE 3A, (a) then Pleads P which in turn leads P by 120 steps. A vector diagram is shown inFIGURE 4. The direction of rotation of the vectors reverse due toreversal of motion. Consequently, a forward motion results in a vector Awhich lags B by 90, while a reverse motion results in vector A leading Bby 90.

The block diagram, FIGURE 2, shows an electronic Schmidt trigger circuit11, which squares the sinusoid A into a square wave A FIGURE 3(0), whicha pulse former 12 converts to a pulse A FIGURE 3(e), at the point ofzero crossover of the square wave. Similarly, square wave generator 13provides square wave B and pulse former 14 provides pulses B The waveshapes of A A B and E are shown in FIGURES 3(c) through 3(f). Anothersummation network 15 adds the signals from the square Wave generatorsand the pulse formers to result in the following:

(I) A +B,,: FIGURE 3gHere the square of A is added to the pulse of B.Consequently, if the direction of movement is forward as in FIGURE 3g,then the pulse is above the square wave with a high positive value. Onthe other hand, if the direction is reversed, then the pulse is belowthe square wave, FIG. 3A(g).

A signal level detector 16 is connected to network 15 so that only thosesignals which are above the square wave are passed to the conventionalcounter 17. The counter contains two inputs, add and subtract, the firstallows it to add a digit per pulse, while the other subtracts a digitper pulse. The signal A -l-B is connected to the add portion of thecounter, consequently, when the signal is above the level shown inFIGURE 3g, the counter adds a digit. When the motion is reversed, nopulse is present above the triggering level, since all pulses presentwithin the A,l-B output will be below the level of the square wave.

(II) A -B This signal is allowed to trigger the subtracting section ofthe counter, as shown in FIGURE 3A(k). Here, if the motion is forward,the signals are below the square wave, and no pulse is allowed totrigger the counter. On the other hand if the direction is reversed,then A B results in a signal above the square Wave, FIGURE 3A(k), whichexceeds the triggering levels so that now a digit is subtracted.

It may be seen from the above that if only A -i-B is connected to theadd section while A B is connected to subtract side, then one pulse isdeveloped per fringe, and reversible counting is achieved.

The configuration of the detector array allows the increase of thenumber of counts per fringe. Thus, the add section of the counter can beconnected to (A +B (B A (-A B and (B A FIGURES 3g, 3h, 31', and 3resulting in four counts per fringe. In addition to the above, thesubtract function can similarly be actuated by (A B (B -l-A (-A +B and(B A FIGURES 3k, 3!, 3m, and 3n.

Further increase in sensitivity is possible by the inherent capabilityof the three photomultiplier scheme. Thus, as shown above it is possibleto obtain four pulses per fringe which are located apart. It is alsopossible to obtain twelve counts per fringe as shown in FIGURE 2, wherethe spacing between counts is 30". This is achieved by comparing thevalue of with that (P P )=D are provided by summation network 10a. Thesevectors are shown to be in quadrature in FIGURE 4. Consequently, fouradd pulses or four subtract pulses can be developed in the same mannerdescribed previously, by squarers 11a and 11b, pulse formers 12a and12b, network 15a and signal level detector 16a.

The third quadrature combination is P1 P2 (P, 2 -E with (P -P )=F, whichare provided by summation network 101), which also results in fourpulses per fringe by means of squarers 11c and 11d, pulse formers 12cand 12d, network 1512, and signal level detector 16b. The total maximumnumber of pulses per fringe formed by combining A & B, C & D, E & F, are12; any selection from 1 to 12 counts per fringe are possible by merelyconnecting the proper outputs to the add-subtract counter 17.

The versatility of the above scheme as well as the independence of thecounting on light levels and contrast results in a unique detectionscheme having great value in instrumentation and measurement.

Therefore, the measuring interferometer of the present invention withits precision optical components produces optical fringes developed byan internal light source. Counting a number of fringes determinesdistance traveled. The wavelength used is produced by a Hg-198 lightsource with a filtered frequency of 5461 angstroms.

The stability of measurement with the present invention is assured byunique method of mounting the optics that do not require adjustmentafter extended usage. A split phase, three photomultiplier detectiongives highly accurate reversible sensing of fringes, independent ofreasonable variation of input voltage, environmental conditions, andtime of operation.

For operation, the sensor is normally mounted stationary. A travellingtetrahedral prism is mounted on the moving body whose location it isdesired to determine. As the body moves, light wave fringes are formedand are counted, between the tetrahedral prism and the sensorrepresenting its exact displacement to a repeatable accuracy.

Automatic optical fringe counting is achieved by a novel detectionscheme using three photomultipliers. The combination of optics andelectronics assures that accurate measurements are obtained independentof vibrations or overshoot of the measured table. A numericalunambiguous output is developed in a counter which may have a visibledisplay calibrated in increments of the order of ten millionths of aninch. The counter can be used for automatic readout, on punch tape,punch card or magnetic tape.

The present invention is a universal system adapted to measure eitherdistance or angles. If desired, angular rates or linear rates over timemay also be measured to an unprecedented accuracy.

I claim:

1. In a phase measuring system,

means to provide a phase reference signal,

means to provide an adjustable phase signal,

means to measure phase shift of said adjustable phase signal withrespect to said reference signal by counting fractional wavelengthscomprising,

means in the path of said adjustable signal to produce first, second andthird signals from said adjustable phase signal, said first, second andthird signals being 120 apart in phase,

means in the paths of said signals to combine said first,

second, and third signals with said phase reference signal to providefirst, second, and third combined signals,

means in the paths of said signals to detect said first,

second and third combined signals and provide detected signals,

first summation network means connected to said detector means tocombine said detected signals to provide a first pair of fourth andfifth sine Wave signals separated by 90 and a second pair of sixth andseventh sine wave signals separated by 90 in phase, said pairs ofsignals being separated by 120,

means connected to said combining means to make square waves of saidfourth, fifth, sixth and seventh signals,

means connected to said square wave means to differentiate said fourth,fifth, sixth, and seventh signals to provide pulses, summation meansconnected to said square wave means and said pulse means to combine saidsquare wave signals and said pulses to provide add signals when saidadjustable phase signal moves in phase in one direction with respect tosaid phase reference signal and to provide subtract signals when saidadjustable phase signal moves in the opposite direction,

and add-subtract counter means connected to said second summationnetwork means.

2. In a phase measuring system,

means to provide a phase reference signal,

means to provide an adjustable phase signal,

means to measure phase shift of said adjustable phase signal withrespect to said reference signal by counting fractional wavelengthscomprising,

means in the path of said adjustable signal to produce first, second,and third signals from said adjustable phase signal, said first, second,and third signals being 120 apart in phase,

means in the paths of said last signal to combine said first, second,and third signals with said phase reference signal to provide first,second, and third combined signals,

means in the paths of said last signal to detect said first,

second, and third combined signals and provide detected signals,

first summation network means connected to said detector means tocombine said detected signals to provide a first pair of fourth andfifth sine wave signals separated by a second pair of sixth and seventhsine wave signals separated by 90, a third pair of eighth and ninth sinewave signals separated by 90, said pairs of signals being separated bymeans connected to said last means to make square waves of said fourth,fifth, sixth, seventh, eighth and ninth signals,

means connected to said last means to differentiate said fourth, fifth,sixth, seventh, eighth and ninth signals to provide pulses,

second summation network means connected to said square wave means andsaid pulse means to combine said square wave signals and said pulses toprovide add signals when said adjustable phase signal moves in phase inone direction with respect to said phase reference signal and to providesubstract sig nals when said adjustable phase signal moves in theopposite direction, and

add-substract counter means connected to said last means to count saidpurse signals thereby measuring said phase shift of said adjustablephase signal.

3. In a cycle counting means comprising,

means to provide a phase reference signal,

means to measure phase shift of said adjustable phase signal withrespect to said reference signal by counting fractional wavelengthscomprising,

means connected to receive said adjustable phase signal and to producefirst, second and third signals from said adjustable phase signal, saidfirst, second, and third signals being 120 apart in phase,

means connected to said last means to combine said first, second, andthird signals with said phase reference signal to provide first, secondand third combined signals,

means connected to said last means to detect said first,

second and third combined signals and provide detected signals,

first summation network means connected to said last means to combinesaid detected signals to provide a plurality of pairs of sine wavesignals separated by 90 in phase, each of said pair of signals beingseparated by 120,

means connected to said last means to make square waves of said pairs ofsignals,

means connected to said last means to provide pulses from said pairs ofsignals,

second summation network means connected to said square wave and pulsemeans to combine said square wave signals and said pulses to provideplus signals when said adjustable phase signal moves in one directionwith respect to said phase reference signal and to provide minus signalswhen said adjustable phase signal moves in the opposite direction, and

plus-minus counter means connected to said last means to count saidpulse signals thereby counting fractional cycles of phase shift.

References Cited by the Examiner UNITED STATES PATENTS 2,835,445 5/ 1958Stephans 235-92 3,127,465 3/ 1964 Stephans 8814 FOREIGN PATENTS1,074,643 2/ 1960 Germany.

WALTER L. CARLSON, Primary Examiner.

P. F. WILLE, Assistant Examiner.

3. IN A CYCLE COUNTING MEANS COMPRISING, MEANS TO PROVIDE A PHASEREFERENCE SIGNAL, MEANS TO MEASURE PHASE SHIFT TO SAID ADJUSTABLE PHASESIGNAL WITH RESPECT TO SAID REFERENCE SIGNAL BY COUNTING FRACTIONALWAVELENGTHS COMPRISING, MEANS CONNECTED TO RECEIVE SAID ADJUSTABLE PHASESIGNAL AND TO PRODUCE FIRST, SECOND AND THIRD SIGNALS FROM SAIDADJUSTABLE PHASE SIGNLA, SAID FIRST, SECOND, AND THIRD SIGNALS BEING120* APART IN PHASE, MEANS CONNECTED TO SAID LAST MEANS TO COMBINE SAIDFIRST, SECOND, AND THIRD SIGNALS WITH SAID PHASE REFERENCE SIGNAL TOPROVIDE FIRST, SECOND AND THIRD COMBINED SIGNALS, MEANS CONNECTED TOSAID LAST MEANS TO DETECT SAID FIRST, SECOND AND THIRD COMBINED SIGNALSAND PROVIDE DETECTED SIGNALS, FIRST SUMMATION NETWORK MEANS CONNECTED TOSAID LAST MEANS TO COMBINE SAID DETECTED SIGNALS TO PROVIDE A PLURALITYOF PAIRS OF SINE WAVE SIGNALS SEPARATED BY 90* IN PHASE, EACH OF SAIDPAIR OF SIGNALS BEING SEPARATED BY 120*, MEANS CONNECTED TO SAID LASTMEANS TO MAKE SQUARE WAVES OF SAID PAIRS OF SIGNALS, MEANS CONNECTED TOSAID LAST MEANS TO PROVIDE PULSES FROM SAID PAIRS OF SIGNALS, SECONDSUMMATION NETWORK MEANS CONNECTED TO SAID SQUARE WAVE AND PULSE MEANS TOCOMBINE SAID SQUARE WAVE SIGNALS AND SAID PULSES TO PROVIDE PLUS SIGNALSWHEN SAID ADJUSTABLE PHASE SIGNAL MOVES IN ONE DIRECTION WITH RESPECT TOSAID PHASE REFERENCE SIGNAL AND TO PROVIDE MINUS SIGNALS WHEN SAIDADJUSTABLE PHASE SIGNAL MOVES IN THE OPPOSITE DIRECTION, AND PLUS-MINUSCOUNTER MEANS CONNECTED TO SAID LAST MEANS TO COUNT SAID PULSE SIGNALSTHEREBY COUNTING FRACTIONAL CYCLES OF PHASE SHIFT.